Pictured above are the orbits of the over 1,000 known Potentially Hazardous Asteroids (PHAs). These documented tumbling boulders of rock and ice are over 140 meters across and will pass within 7.5 million kilometers of Earth -- about 20 times the distance to the Moon. Although none of them will strike the Earth in the next 100 years -- not all PHAs have been discovered, and past 100 years, many orbits become hard to predict.

The solar-electric propulsion thruster pictured above ditches burning chemical fuel for xenon ions accelerated by an electric field generated using solar panels. This provides a steady stream of ions that can slowly nudge a spacecraft to high speeds with minimal fuel, making it a good choice for long-range missions.